This invention relates to a semiconductor device having external electrode members that are brazed to electrodes formed on a semiconductor substrate, and also to a method of fabricating such a semiconductor device.
Generally, semiconductor devices have electrodes formed at predetermined portions on the surface of a semiconductor substrate and external electrode members for connecting the electrodes to the outside. The shapes of the external electrode members are diversified such as line-like, sheet-like, foil-like, and so forth. When these external electrode members are connected to the electrodes on the semiconductor surface with two-dimensional expansion, these conductive elements are mostly connected to one another by brazing or soldering.
If, in this case, the exothermic quantity on the semiconductor substrate is relatively great for handling relatively large power or for other reasons, a brazing material having a relatively high melting point and high resistance to heat fatigue is used as the brazing material. For instance, it has a composition of 95 wt% Pb and 5 wt% Sn.
However, a problem occurs from time to time that the use of a brazing material having a high melting point fails to satisfy other characteristics of a semiconductor device or characteristics required in fabricating the semiconductor device. This point will be described more definitely.
The inventors of the present invention previously proposed a semiconductor device which uses a metallic foil bonded to a resin tape as the external electrode members. In the semiconductor devices of this kind, the electrodes on the semiconductor substrate and a metallic foil having substantially the same shape as that on the electrodes of the semiconductor substrate are disposed so as to oppose one another via a brazing material and the brazing material is then fused so as to bond the foil and electrodes together. According to this arrangement, the metallic foil supplements the thickness on the electrodes of the semiconductor substrate, reduces the resistance of the electrodes as a whole and increases the current feed quantity even if the electrodes on the semiconductor substrate are thin and micro-small and hence, even if they alone can not provide a sufficient feed current quantity.
If a solder having a high melting point is used for brazing such micro-small electrodes, however, the following problems are yet to be solved in conjunction with the fabrication of the device. First, since the solder having a high melting point has a high treating temperature, the atmosphere for the treatment must be adjusted to be a reducing atmosphere. In a non-reducing atmosphere, the solder and the electrode material on the semi-conductor substrate would be oxidized because the treating temperature is high and a connection having high strength could not be obtained. It has been difficult from the aspect of fabrication of the device to adjust the surrounding atmosphere to a specific type of atmosphere and to connect the electrodes having a micro-small shape at a high temperature with a high level of accuracy.
It has been clarified that even if the above-mentioned problem were solved, another problem develops in that resistance to heat fatigue is not sufficiently high if a connection is made in accordance with the conventional method using the solder having a high melting point. This is clarified by the inventors of the present invention in the course of evaluation of the resistance to heat fatigue. It has been confirmed that the strength of even a solder having a high melting point drops to an unpractical level after a large number of heat cycles are applied to the test-piece.